They are much better rooted in a free draining mix. Water will massively increase the chances of them just rotting off. I would remove the yellow leaves, trim the stems at the bottom, leave the cut ends to dry out, then insert into mix. Here's how.

Floral is right. Put them in container with a free draining mix. These root quite easily. In late spring I trim all the leggy growth of my geraniums when putting them outside. I just stick the trimmings all around in the garden and most will strike roots in a few weeks with no extra protection. And I end up with nearly instant flower garden. Some of them even grow so fast that they are bigger than the mother plant in the hanging basket.

I've heard that letting them callous over a bit before sticking can be helpful in preventing rot.... however I do not speak from experience....

Andrea: That may be a better idea but I have so far done only the opposite. But I also do this in late spring or early summer and so my successes may have something to do with that. They are quite resilient and so I think it should be fairly easy.

It's the middle of winter here in Sydney hmm...

I rooted one in water this spring, it took about 6 weeks before I got a root on it. I put it in soil the next day and it's done great, has recently started flowering.

Went on holiday for 10 days and look – I now have roots! Planting soon! Thanks everyone! I will let my dad know this worked ; )